Several large clinical trials of angiotensin converting enzyme (ACE) inhibitors have demonstrated the critical role of the renin angiotensin system (RAS) in the pathogenesis of heart failure. These trials also indicate an unexplained heterogeneity in response to ACE-inhibitor therapy. The general hypothesis to be tested in this application is that there is variation in ACE activity in the falling human heart. An in vivo protocol using intracoronary 123I-Ang I will be used to study Ang II formation in the intact failing human heart.